[cfe-commits] [PATCH] Smarter implicit copy-construction/copy-assignment.

[Lang Hames]

Sure thing. I'm working on an update at the moment - I'll roll this in.

Cheers,
Lang.


On Thu, Feb 7, 2013 at 3:25 PM, John McCall <rjmccall at apple.com> wrote:

> On Feb 5, 2013, at 6:45 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>
> On Tue, Feb 5, 2013 at 5:31 PM, Lang Hames <lhames at gmail.com> wrote:
>
>> Hi John, Richard,
>>
>> Here's the updated patch for smarter copy-assignment/copy-construction
>> for structs with Non-POD members.
>>
>> Richard - I've added test cases for volatile fields, inner classes, array
>> members, pod-like members, bitfields and volatile fields.
>> I've also pulled the naming scheme in line with the coding standards. I
>> made sure I use ' &' rather than '& ' where appropriate.
>>
>> John -
>>
>> 1) Qualifiers are now being grabbed once.
>> 2) I now fall back on existing codegen for when GCMode != NonGC.
>> 3) Made sure I round-up to the nearest byte when copying bitfields.
>> 4) I've tried to address the style issues you raised.
>> 5)
>>
>>
>> > +      // Bail out on non-POD, not-trivially-constructable members.
>> > +      if (!FieldType.isPODType(GetCGF().getContext()) &&
>> > +          (!CE || !CE->getConstructor()->isTrivial()))
>> > +        return false;
>> >
>> > If you're calling a trivial copy/move constructor, it does not matter
>> > whether the type is POD.  Test case: give a field a type with a trivial
>> > copy constructor and non-trivial destructor, or more realistically a
>> > non-trivial default constructor.
>>
>> Note that this is the test for bailing out. The POD check needs to be
>> there for primitive types, for which CE == 0, otherwise we'd decide that we
>> couldn't memcpy primitive fields.
>>
>
> Just because the type is POD, doesn't mean the selected constructor is
> trivial.
>
>
> The point is that Lang's code was gating on *both*, which is clearly
> unnecessary in the case of class types.
>
> Counterexample:
>
> struct S { S() = default; template<typename T> S(T&&); S(const S&) =
> default; };
> struct U { U(); S s; };
> static_assert(__is_pod(S), "");
> U u = static_cast<U&&>(U()); // static_cast to disable copy elision
>
> S is POD, but U's move constructor calls S's constructor template.
>
> I think this is what you mean:
>
>   CE ? CE->getConstructor()->isTrivial() :
> FieldType.isTriviallyCopyableType(Ctx)
>
>
> Mmm.  This would actually drop members of reference type, which really can
> be safely memcpy'ed in a copy constructor.
>
> On the other than, it would also drop members of atomic type, which really
> shouldn't be memcpy'ed.  So there's that.
>
> Lang, I think using Richard's proposal and then specifically opting-in a
> couple of extra cases (e.g. reference types) would make a lot of sense.
>
> John.
>
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